Coaxial cables are widely used for transmitting high frequency electrical signals, such as for transmitting modulated video signals via cable television networks. A coaxial cable typically includes a center conductor, a surrounding layer of dielectric material, and a surrounding tubular outer conductor. An overall protective jacket may also be included surrounding the outer conductor.
Coaxial cable is typically manufactured, such as by the assignee of the present invention, by a multi-step process. A cable core is typically first prepared by extruding a dielectric material layer surrounding an advancing elongate center conductor. The core is then advanced along a predetermined path of travel as a flat conductive tape is advanced adjacent the core. The core and surrounding tape pass through a series of tube forming rolls which progressively shape the flat tape into a generally cylindrical shape having a longitudinally extending gap of predetermined width between adjacent tape edges.
Downstream from the tube forming rolls, the gapped cylindrical tape passes through a weld coil which imparts radio frequency (RF) energy to heat the tape, particularly at the adjacent longitudinal edges. Downstream from the weld coil is a pair of opposing weld rolls which are spaced to cause the heated edges of the tape to press together and form a welded seam including a slightly outwardly protruding weld bead. This protruding bead is removed by a fixed scraper blade downstream from the weld rolls.
The thus-formed outer conductor is slightly larger than the contained core. Accordingly, downstream from the scraper blade, the outer conductor passes through a series of sinking dies which progressively reduce the diameter of the outer conductor until the outer conductor snugly engages the core.
The tube forming rolls are typically rotatably mounted to a frame as are the downstream weld rolls. The tube forming rolls typically have an adjustable spacing between rolls in each pair. The spacings determine how much the tape is worked, or reduced in width, as it passes through the tube forming rolls. The spacing or gap at the last set of rolls of the tube forming rolls is of particular importance in producing higher quality coaxial cable.
Similarly, the pressure applied by the weld rolls to the heated longitudinal edges of the tape may also have a considerable impact on the quality of the longitudinal weld in the outer conductor. The pressure is determined by the spacing between the weld rolls which is also typically adjustable. The pressure serves to mobilize any oxidation on the edges of the tape and remove any irregularities along the untrimmed edges. Imperfections or defects in the outer conductor, and especially periodically occurring defects, may greatly affect high frequency signal performance of the coaxial cable.
The spacings between the tube forming rolls and the weld rolls also affects wear of these components. In particular., if the spaces are too narrow, unnecessary tooling wear may result. The spacings between the exit tube forming rolls and the weld rolls are typically determined by trial and error. Moreover, preferred settings may vary from operator to operator.
Several attempts have been disclosed to measure the external diameter of a longitudinally welded tube to control the quality thereof. For example, U.S. Pat. No. 4,287,402 to Hentzschel et al., discloses an apparatus which measures the upsetting path by measuring a reduction in circumference of the tube from a first point, where the edges are spaced apart and a second point, where the edges are joined downstream from the welding location. The measurement is made by placing markings on respective opposing edge portions of the tube and detecting a change in the separation between the markings by optical scanning means. The measurement signal is compared to a reference value to thereby generate a signal to control the spacing between the weld rolls.
An alternative approach is disclosed in U.S. Pat. No. 4,734,981 to Ziemek which discloses an apparatus for forming a welded metal tube from a metal strip wherein the thickness of the strip is measured and a capstan station provides a pulling or pushing force responsive to the measured thickness to provide greater uniformity of the tube. Similarly, U.S. Pat. No. 2,819,369 to Dexter, Jr. discloses a dimension gauging system including a measuring station which measures the thickness dimension of the material in sheet form. A signal representative of the thickness is stored in memory and, if the thickness of the material passing through the measuring station changes, an imbalance is created in the memory which triggers a resultant change in heat applied at the weld point based upon the thickness in the material.
Coaxial cable preferably has a fairly large bandwidth, on the order of 1 GHz or more. Accordingly, it is desirable to manufacture the coaxial cable to fairly exacting tolerances to obtain uniform high frequency signal transmission characteristics. In particular, the quality of the longitudinal weld in the outer conductor and the diameter of the outer conductor affect cable quality.
In the past, trial and error, as well as operator experience has been used to make the tooling adjustments to produce the cable. Unfortunately, the weld strength of the coaxial cable may typically be tested by destructive testing of samples taken from both ends of the cable reels after it has been manufactured. Accordingly, there is no real time operator feedback based upon weld strength testing. Rather, the operator makes adjustment to the forming roll spacings, and weld roll spacings, for example, based upon visual observation and his experience. For example, the look and feel of a spark plume created as the seam is formed at the weld rolls may be used to adjust the spacing of the weld rolls. More importantly, as operators are rotated, high quality and product uniformity may be difficult to achieve.